The present invention relates to a metal halide discharge lamp, a lighting device for a metal halide discharge lamp, and an illuminating apparatus using a metal halide discharge lamp.
A metal halide discharge lamp comprises a light-emitting tube provided with a pair of electrodes arranged to face each other. A rare gas, a halide of a light-emitting metal, and mercury are sealed in the light-emitting tube to form the metal halide discharge lamp. The discharge lamp of the particular construction exhibits a relatively high efficiency, and high color rendering properties and, thus, is used widely.
The metal halide discharge lamp is classified into a short arc type and a long arc type. The short arc type metal halide discharge lamp is used in projectors such as a liquid crystal projector in which light rays emitted from a lamp are collected so as to be projected onto a screen, and an overhead projector, and is also used for illumination of shops in the form of down light and spot light. Also, a small short arc type metal halide discharge lamp has come to be used in recent years as a headlamp of a vehicle in place of a halogen lamp.
As described in, for example, Japanese Patent Disclosure (Kokai) No. 2-7347, it is absolutely necessary to seal about 2 to 15 mg of mercury in a metal halide discharge lamp used as a headlamp of a vehicle.
On the other hand, Japanese Patent Disclosure No. 3-112045 discloses a metal halide discharge lamp which does not necessitate the sealing of mercury. In this prior art, a rare gas such as helium or neon is sealed in the lamp at a pressure of 100 to 300 Torr in place of mercury so as to obtain a desired lamp voltage. Since the atom of each of these rare gases has a small radius, the rare gas permeates through a quartz glass and, thus, the hermetic vessel of the lamp is formed of a transparent ceramic material.
On the other hand, the long arc type metal halide discharge lamp is used mainly for the general illumination purposes. For example, the discharge lamp of this type is used as an illumination equipment for a high ceiling, a light projector, a street lamp and as an illumination equipment for roads. Further, a metal halide discharge lamp which generates ultraviolet rays is used for the manufacture of a photo-setting synthetic resin or ink. The metal halide discharge lamp used for this purpose is also of a long arc type.
In any of the short arc type and long arc type metal halide discharge lamps which have been put to practical use nowadays, it is absolutely necessary to use mercury, because, in the metal halide discharge lamp, mercury serves to obtain a desired lamp voltage so as to maintain satisfactory electric properties.
To be more specific, where, for example, the lamp voltage is unduly low, the lamp current must be increased in order to obtain a desired lamp input. In this case, problems are brought about that the current capacities are increased in the related facilities such as the lighting device, illuminating device and wiring. Also increased is the heat generation.
On the other hand, where the lamp current is unduly high, the electrode loss is increased, leading to a low lamp efficiency. To be more specific, the electrode drop of the metal halide discharge lamp is constant for each lamp. As a result, if the lamp voltage is unduly low, the lamp current must be increased for making up for the low-lamp voltage, with the result that the electrode loss is increased in proportion to the lamp current so as to lower the lamp efficiency.
As pointed out above, it is generally advantageous in a discharge lamp to set the lamp voltage at a value as close to the input voltage of the lamp as possible, i.e., as high as possible, as far as the arc does not disappear.
Let us describe the reason why the mercury sealing was required in the conventional metal halide discharge lamp while giving consideration to the lamp voltage with reference to FIG. 1. As shown in the drawing, the lamp comprises a hermetic vessel 1, a pair of electrodes 2, 2, and lead wires 3, 3. The lamp voltage V1, which denotes the voltage between the lead wires 3, 3 when the metal halide discharge lamp is lit, can be represented by formula (1) given below:V1=E×L+Vd  (1)
where E is a degree of potential inclination of the plasma between the electrodes, L is a distance between the electrodes, and Vd is an electrode drop.
The potential inclination degree E of the plasma can be represented by formula (2) given below:E=I/2π∫σrdr  (2)
where I is a lamp current, σ is an electrical conductivity of the plasma, which is a function of temperature T, and r is a distance of an optional point from the center in the radial direction.
If a substance A is supposed to be present within the discharge space during the lighting of the metal halide discharge lamp, the electrical conductivity σ of the substance A at temperature T is given by formula (3) below:σ=C·NE/(T1/2·(NA·Q))  (3)
where C is a constant, NE is an electron density, NA is a density of the substance A, and Q is a cross section of impingement of the electron against the substance A.
As apparent from formula (1), the lamp voltage V1 is increased with increase in the potential inclination degree E and with increase in the distance L between the electrodes. On the other hand, formula (2) indicates that the potential inclination degree E is increased with decrease in the electrical conductivity σ and with increase in the lamp current I. Further, formula (3) indicates that the electrical conductivity σ is decreased with decrease in the electron density NE and with increase in the density NA of the substance A and in the impinging cross section Q. It follows that, where the distance L between the electrodes and the lamp current I are set constant, the conditions of the substance A, under which the lamp voltage V1 is increased, are that the substance A is unlikely to be ionized to diminish the value NE, that the substance A has a high density within the lamp to increase the value NA, and that substance A has a large cross section Q of the electron impingement.
It should be noted that mercury has a very high vapor pressure, i.e., 1 atmosphere at 361° C., is unlikely to be ionized, and has a large cross section of the electron impingement. It follows that a desired lamp voltage can be easily obtained by controlling the sealing amount of mercury in accordance with the size of the lamp. In other words, mercury is sealed in the conventional metal halide discharge lamp because a desired lamp voltage can be obtained easily.
It should be noted in this connection that, in the case of a metal halide discharge lamp, it is necessary to set the mercury vapor pressure higher with miniaturization of the lamp in which the distance L between the electrodes is shortened in order to ensure a desired lamp voltage. For example, in a small short arc type metal halide discharge lamp whose light-emitting tube has an inner volume of 1 cc or less, the mercury vapor pressure during lighting of the lamp is as high as at least 20 atmospheres.
Let us describe the problems which are brought about where mercury is sealed in a metal halide discharge lamp and the problems which are brought about where mercury is not sealed in the conventional metal halide discharge lamp.